Back in spring I bought a couple of PQ1253 modules on Ebay. I didn't know much about Calrec, apart from some good reports from a friend of mine who does outside broadcast stuff for the BBC.I think what initially attracted me was the use of the lundahl input transformers. I have had great 'sonic' experiences with the Focusrite ISA430, which also uses a Lundahl input transformer, albeit a different type.However, when the modules arrived and I opened them up I was very impressed with the elegant design. Lots going on but so well laid out... and most of all serviceable!Possibly an unfair comparison, but I tried to repair one of my ISA430s whilst on tour, nightmare...

My initial plan was to rack the 2 modules and so I purchased an RG 1653 Module on Ebay, just to recycle the LL1517 output transformers. Once again I was so impressed with the layout and the design that it seemed criminal to scrap all the other stuff just for a couple of transfomers...

Also, I have always found that using something, when orientated in a different way to how it was designed always compromises how usable it is. That could just be my live background, but either way it was justification enough for me to buy more PQ modules.

Subsequently the idea dawned on me to build myself a M-series sidecar, and to that end have acquired several more M-series modules:

6 x PQ 12538 x RC 16521 x RG 1653

Gareth of GJC Designs has been very helpful in talking me through the functions of these modules and I am slowly refining my plan for building the console.

With the PQ 1253 linked an RC 1652 module I can achieve a TX Mic and Line in (with an additional external switch) TX Balanced direct out, TX balanced Insert send, and a balanced insert return.I understand that the RC unit also has a fader pins on its edge connector, so I am planning on attaching this via a VCA and using some 10K lin P&G faders I also picked up cheap on ebay.

I have an idea for metalwork that I will continue to refine, but my general plan is to manufacture a motherboard to take the RC modules which will carry all the buses. Depending on cost the PQ modules might be mounted onto individual smaller PCBs.I am going to look at building something 16:8:2 with the ability to extend in channel blocks of 8.The principle use will be to be a front end for Protools, and obviously it would be handy to use for analogue mixing, so the inclusion of groups seems justifiable, especially given the low cost of these modules at the moment.

One thing I am aware of is that I have no summing amps for the buses, hopefully someone here will be able to advise the best way of achieving this (card numbers etc).

Anyway, please forgive my rambling introduction. I appreciate this is an ambitious project, but then again I have designed and built our house (its taken 5 years and it still isn't finished but that's not the point...)I hope documenting my progress here will be a good use of this forum's resources.

I found some verostrip tall enough to fit a pair of DIN 41612 connectors and set about building myself a test jig to hold a PQ 1253 module and an RC1262 module.Using a few pointers from Doug Selfs Small Signal Audio Design I built a simple +-15V supply for the audio rails, and using an old Sony camera charger I made an 8V, and 5V supply.Having fired it up and metered the relevant pins of the DIN 41612 connectors before mounting any modules, all the supply points appeared to be correct.Mounting a single P1253 and applying power resulted in a high pitch oscillating noise(before any kind of input or output had been connected). Reminded me of relay noises when doing gigs where the voltage dropped from 230 to 140 in the space of 20 minutes(Tajikistan, long story) so I checked the power rails again and realised I hadn't bonded the earths of the 2 separate power supplies. One bond later and the unit powered up without the high pitched whining noise. And so to test audio: well I cant seem to be able to switch the PQ unit between line and mic input irrespective of the link between Amp board pin 1 and EQ board pin 8. Is this just a function on the other PQ modules and not the 1253?At this point the little boy arrived back from school, and I am off on tour tomorrow for a month, so I had to cut short the testing, what I did grasp was that the EQ sounded quite special, I wanted to test the response with Fuzzmeasure, but the results where quite odd. Threw me for a bit, but then remembered what I had heard so probably a problem with setting up Fuzzmeasure rather than the general setup.Photos to follow when I get a chance.

Good news!I am not sure how the mic/line switching works on these modules either, on the patchbay on my desk (the whole outside broadcast truck presumably designed for mic and line inputs) there is only one input. I have wondered if the red input level knob switches to line level at the lowest setting and all three, line, mic and alt mic inputs, are wired to the same patch point. Sounds like progress is being made anyhow.If I get some manuals/schematics that explain things I'll let you know.

So now that touring and Christmas are out of the way, its time for me to turn my attention back to my sidecar project.Since my last post I have managed to acquire a few more modules and cards and now my inventory is as below.

I have decided to approach things from a more specification orientated side.The principle uses I would have for this sidecar would be for Tracking into Protools/multitrack and for partial mixing again from Protools.On the Tracking side building a frame that consisting of 8 PQ1253s and 8 RC1652s would allow me a mic in, balanced insert send and return and a direct out all balanced via LL1517s.From the mixing side, it would appear that the PQ modules I have do not have a line input, however it wouldn't be that hard to make a few line in cards to compliment each PQ, these cards could also incorporate an LED bargraph, line/Mic select, 48V and sit at the head of the channel strip.Adding a fader VCA for each RC module in the form of a THAT 2181 chip should be achievable, possibly modifying the Post Fader amp to be unity gain and to do the I/V conversion.With the generous help of some of this sites members I have now discovered that the RG1653 modules incorporate the summing amps for the group buses and are also stereo, so ideal for mixing Stems.In the most basic of setups I could use the insert sends as a Stem feed for Protools, although I would probably end up using analogue inserts for any stems mixed or tracked, so a set of dedicated balanced outs would be very useful.The same VCA fader mod for the RC module would work for the RG module.Referring to the schematic for the XL1948, I could use 2 to sum and balance the IND, Main and MCF feeds...So the next task would be to figure out getting the AFL/PFL bus to function correctly, given that I have a pretty good idea of what I want to achieve with regards to a monitoring section suited for mixing, I think I will write a specification and then set about achieving that using the various bits of Calrec modules I have left over plus adding a few extra bits (I am very keen on having a stepped monitor level out control).

The one thing I do seem to be missing is the Aux/Foldback master section. Looking at pictures of M-series consoles there appears to be a master out section located near the main out, I don't know if this module actually did the bus summing for the aux buses or if that was located on a card in the rear of the console.I have no idea what the function of the 3 x LN1660,1 x MB1743or the 1 x MB2221 are, so if anyone could shed any light that would be much appreciated. Also forgive my ignorance but I was slightly surprised by the use of 10K lin pots for the Aux send levels, I would assume that this wouldn't give a very even range of rotational gain control. Was the summing amp configured to compensate for this? I cant quite get my head around if this would work or not, I think it would but I haven't done any calculations.

Referring to the few schematics and just the general construction, I have to mention my admiration for the engineering that went into these consoles, the building block nature of them means that it seems possible to build a custom configuration without much trouble at all(obviously while keeping a very judicious eye on the various operating levels, virtual earth points, and ground cancelling stuff )...Of course I could be about to come crashing back down to earth at any minute!

The Autocad drawing are going quite well, and I have ordered a few samples of PCB slot guides to try out for locating the modules. Once I have found one that works I will push on towards the CNC fabrication of 10 or 12 module “U-shaped bays” for each size of module. 1 for the PQ, the RC, and I am going to use the SC 1450-6 module front as a template for the line in/metering module.The cross section pieces will then hold the bays in orientation to each over so that I can wrap the console upwards if necessary to improve usability, space etc.Many thanks to Alex, the pics posted of the empty frame gave me loads of clues as to how the convection cooling was achieved, very useful!

My next task is to build a small format mixer consisting of 2 RC modules and 3 RG modules, using stripboard for the motherboard, I will discretely wire the XL cards and see how things go.The signal generator modules will come in very handy for the testing!

Progress!!! good to see.No problem for the pictures, glad they could help.If you need any pictures of any bits in particular just give me a shout and I'll sort it out.

The PQ modules don't have a seperate "line in" on my desk so I'm assuming the red level pot that switches between the 2 pres is also a line level selector. The writing on the faceplate suggests the second click is 0db the first being -10db.I am thinking that the same input being used for line and mic inputs is not a problem for sound quality or the bbc would have demanded seperate inputs.

The aux module next to the monitor section on my desk doesn't contain any amps or much at all infact. There are cards in the back, each card handles 2 signals, ie. card 1 - aux 1 A+B. card 2 - aux 2 + aux 3. etc. They are XL1499 cards.

Dont know about the 3 x LN1660,1 x MB1743or the 1 x MB2221. my desk has one MB2265 which is "AFL C/O Dim" but no other similar numbers.The aux pots have a pretty smooth taper to my ears. If you are near bradford or passing you are welcome to come and have a look/listen in the flesh.Good luck with the project, I did my first bit of proper tracking with mine last night and it has blown me away, it sounds amazing. There is so much detail and clarity it's quite scary, you really hear where you're going wrong.....and right.Well worth all the trouble and expense.

Thats a huge help, I know the XL1499 is like a dual version of the XL1498 (A few differences in signal polarity...). Now I know that, I should be able to test pretty much the complete signal path, best get the soldering iron out!

So I managed to build my motherboard using two strips of Verostrip, mounting them at the appropriate distance apart and the wiring the bus master sends and BSE feeds between the 2 boards.I configured the 2 boards so that the 1st and 2nd would accept RC modules and then the 3rd and 4th slots would accept RG modules. The end of the strips acting as buses had single core wire attached to them to allow easy routing to the XL1498 module via breadboard.I also added wires for the SEP out and Insert Send/Return, for the RC modules and the 2 sets of Insert Send/Return for the Rg modules.I borrowed a few 10K log faders from an DDA Q series that I have sitting under the spare bed.For the time being I have decided to leave the PFL send to fader and return unconnected as I have an idea for a mod to make the AFL and PFL signals route to the appropriate buses from pressing the AFL button on the module. The signal routed to the monitor section can then be selected at the master section.The mod will effectively remove the need for the Alt PA command bus so that can be ignored leaving only the group routing status lines and the AFL relay line.Another piece of stripboard was fitted with DIN 41612 connectors with all but the power lines broken between them. This allowed this board to be used to power the XL 1498 card, and the SC1450 signal generator module.Finally, I mounted the various bits of ratty Power supplies on a single bit of MDF to create a PSU that some of may describe as “Super Ghetto”.

Motherboard with Ghetto style PSU

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The purpose of this “motherboard' was so that I could test levels throughout the signal chain and confirm that the gain structure worked as I expected it too before committing myself to manufacturing the various Motherboards.At this stage I decided to use the unbalanced output from the signal generator into the RC modules PQ return input and trim it as close to the -14dBu, which is the level that the module is expecting to see. As it happened this was only -12dBu so I then measured the level at the fader send confirmed it was the same and then trimmed the fader to give -24dBu. After the fader amp this should return the level to -14 dBu for the rest of the routing.However, at this point a lot of the levels didn't seem to add up, the outputs from the RG module didn't seem to tally with what I was expecting and I started to doubt my understanding and did a few quick changes without really thinking what I was doing....

Populated with one RC and one RG module

motherboard RCRG.jpg (265.06 KiB) Viewed 9394 times

So a cup of coffee and a bit of a chill out later, I returned to the testing but this time just measured signals at various points through the signal chain and tabulated the results. Pretty soon I started to suspect the RG module I had installed so I swapped it out and sure enough the results started to make much more sense. Namely, when I trimmed the RG module fader return to -16dBu and then routed that to the IND, Main, or MCF bus(with the XL1498 card attached to the appropriate bus) the XL card yielded 0dBu as I expected. Attaching the XL1498 to the Aux send bus gave me a nice summed output with the increase of gain expected for the different value bus resistors.A few of the measurements seemed to be out by +- 1.5 dB which puzzled me for a while. But when reviewing the data afterwards I realised I had trimmed the fader return for the RC module at -22dBu rather than -24dBu, this error was corrected for the RG module measurements, due to the fader trim , but it meant that routings direct from the RC modules seemed slightly higher than expected.

Motherboard Full.jpg (223.45 KiB) Viewed 9394 times

So all in all a pretty useful afternoons testing. Next job is to have a look at designing the motherboard in Autocad. I am going to have a go at making the motherboards using a CNC engraving machine that I have access to. Hopefully, I will incorporate this with machining some of the aluminium panels to make up the module bays.I also need to decide how to interlink the RC motherboard with the RG motherboard. I am not sure how it was done on the original M-Series, but I quite like the idea of using a pair of right angle Din 41612 connectors situated at the ends of the boards, this would allow easy expansion in blocks of 8 or 12.

Anyway thats all for now, thankyou to everyone who has offered assistance!